In Apocalypse 3, Larry describes the changes that Perl 6 will make
to operators and their operations. As with all the Apocalypses, only the
new and different are presented -- just remember that the vast majority
of operator-related syntax and semantics will stay precisely as they are
in Perl 5.

For example...

To better understand those new and different aspects of Perl 6 operators,
let's consider the following program. Suppose we wanted to locate a particular
data file in one or more directories, read the first four lines of each
such file, report and update their information, and write them back to
disk.

I was bound under a flattening
star

The first subroutine takes a filename and (optionally) a version number
and a list of directories to search:

sub load_data ($filename ; $version, *@dirpath) {

Note that the directory path parameter is declared as *@dirpath,
not
@dirpath. In Perl 6, declaring a parameter as an array (i.e
@dirpath)
causes Perl to expect the corresponding argument will be an actual array
(or an array reference), not just any old list of values. In other words,
a @ parameter in Perl 6 is like a \@ context specifier
in Perl 5.

To allow @dirpath to accept a list of arguments, we have to
use the
list context specifier -- unary * -- to tell Perl
to "slurp up" any remaining arguments into the @dirpath parameter.

This slurping-up process consists of flattening any arguments that are
arrays or hashes, and then assigning the resulting list of values, together
with any other scalar arguments, to the array (i.e. to
@dirpath
in this example). In other words, a *@ parameter in Perl 6 is
like a @ context specifier in Perl 5.

Editor's note: this document is out of date and remains here for historic interest. See Synopsis 3 for the current design information.

It's a setup

In Perl 5, it's not uncommon to see people using the ||= operator
to set up default values for subroutine parameters or input data:

Of course, unless you're sure of your range of values, this can go horribly
wrong -- specifically, if the variable being initialized already has a
valid value that Perl happens to consider false (i.e if
$suffix
or $last_suffix or $default_suffix contained an empty
string, or the offset really was meant to be zero).

So people have been forced to write default initializers like this:

$offset = 1 unless defined $offset;

which is OK for a single alternative, but quickly becomes unwieldy when
there are several alternatives:

Perl 6 introduces a binary 'default' operator -- // -- that solves
this problem. The default operator evaluates to its left operand if that
operand is defined, otherwise it evaluates to its right operand. When chained
together, a sequence of // operators evaluates to the first operand
in the sequence that is defined. And, of course, the assignment variant
-- //= -- only assigns to its lvalue if that lvalue is currently
undefined.

The symbol for the operator was chosen to be reminiscent of a ||, but
one that's taking a slightly different angle on things.

So &load_data ensures that its parameters have sensible
defaults like this:

$version //= 1;
@dirpath //= @last_dirpath // @std_dirpath // '.';

Note that it will also be possible to provide default values directly in
the specification of optional parameters, probably like this:

...and context for all

As if it weren't broken enough already, there's another nasty problem with
using || to build default initializers in Perl 5. Namely, that
it doesn't work quite as one might expect for arrays or hashes either.

If you write:

@last_mailing_list = ('me', 'my@shadow');

# and later...

@mailing_list = @last_mailing_list || @std_mailing_list;

then you get a nasty surprise: In Perl 5, || (and &&,
for that matter) always evaluates its left argument in scalar context.
And in a scalar context an array evaluates to the number of elements it
contains, so
@last_mailing_list evaluates to 2. And that's
what's assigned to
@mailing_list instead of the actual two elements.

Perl 6 fixes that problem, too. In Perl 6, both sides of an ||
(or a
&& or a //) are evaluated in the same context
as the complete expression. That means, in the example above, @last_mailing_list
is evaluated in list context, so its two elements are assigned to
@mailing_list,
as expected.

Substitute our vector, Victor!

The next step in &load_data is to ensure that each path in
@dirpath
ends in a directory separator. In Perl 5, we might do that with:

s{([^/])$}{$1/} foreach @dirpath;

but Perl 6 gives us another alternative: hyper-operators.

Normally, when an array is an operand of a unary or binary operator,
it is evaluated in the scalar context imposed by the operator and
yields a single result. For example, if we execute:

That is, to apply the operation element-by-element, pairwise along the
two arrays.

Perl 6 makes that possible, though not by changing the list context
behavior of the existing operators. Instead, Perl 6 provides a "vector"
version of each binary operator. Each uses the same symbol as the corresponding
scalar operator, but with a caret (^) dangled in front of it.
Hence to get the one-to-one addition of corresponding credits and debits,
and the list of differences between pairs of sheep and goats, we can write:

This works for all unary and binary operators, including those that
are user-defined. If the two arguments are of different lengths, the operator
Does What You Mean (which, depending on the operator, might involve padding
with ones, zeroes or undef's,
or throwing an exception).

If one of the arguments is a scalar, that operand is replicated as many
times as is necessary. For example:

@interest = @account_balances ^* $interest_rate;

Which brings us back to the problem of appending those directory separators.
The "pattern association" operator (=~) can also be vectorized
by prepending a caret, so we can apply the necessary substitution to each
element in the @dirpath array like this:

@dirpath ^=~ s{([^/])$}{$1/};

(Pre)fixing those filenames

Having ensured everything is set up correctly, &load_data
then processes each candidate file in turn, accumulating data as it goes:

my %data;
foreach my $prefix (@dirpath) {

The first step is to create the full file path, by prefixing the current
directory path to the basic filename:

my $filepath = $prefix _ $filename;

And here we see the new Perl 6 string concatenation operator: underscore.
And yes, we realize it's going to take time to get used to. It may help
to think of it as the old dot operator under extreme acceleration.

Underscore is still a valid identifier character, so you need to be
careful about spacing it from a preceding or following identifier (just
as you've always have with the x or eq operators):

In Perl 6, there's also a unary form of _. We'll get to that a
little later.

Editor's note: this document is out of date and remains here for historic interest. See Synopsis 3 for the current design information.

Don't break the chain

Of course, we only want to load the file's data if the file exists, is
readable and writable, and isn't too big or too small (say, no less than
100 bytes and no more than a million). In Perl 5 that would be:

In Perl 6, the same set of tests can be considerably abbreviated by
taking advantage of two new types of operator chaining:

if (-w -r -e $filepath and 100 < -s $filepath <= 1e6) {...

First, the -X file test operators now all return a special object
that evaluates true or false in a boolean context but is really an encapsulated
stat
buffer, to which subsequent file tests can be applied. So now you can put
as many file tests as you like in front of a single filename or filehandle
and they must all be true for the whole expression to be true. Note that
because these are really nested calls to the various file tests (i.e. -w(-r(-e($filepath)))),
the series of tests are effectively evaluated in right-to-left order.

The test of the file size uses another new form of chaining that Perl
6 supports: multiway comparisons. An expression like 100 < -s $filepath
<= 1e6 isn't even legal Perl 5, but it
Does The Right Thing in Perl 6. More importantly, it short-circuits
if the first comparison fails and will evaluate each
operand only once.

Open for business

Having verified the file's suitability, we open it for reading and writing:

The : mode=>'rw' is an adverbial modifier on the open.
We'll see more adverbs shortly.

The $! variable is exactly what you think it
is: a container for the last system error message. It's also considerably
more
than you think it is, since it's also taken over the roles of
$?
and $@, to become the One True Error Variable.

Applied laziness 101

Contrary to earlier rumors, the "diamond" input operator is alive and
well and living in Perl 6 (yes, the Perl Ministry of Truth is even now
rewriting Apocalypse
2 to correct the ... err ... "printing error" ... that announced <>
would be purged from the language).

So we can happily proceed to read in four lines of data:

my ($name, $vers, $status, $costs) = <$fh>;

Now, writing something like this is a common Perl 5 mistake -- the list
context imposed by the list of lvalues induces <$fh> to read
the entire file, create a list of (possibly hundreds of thousands of) lines,
assign the first four to the specified variables, and throw the rest away.
That's rarely the desired effect.

In Perl 6, this statement works as it should. That is, it works out
how many values the lvalue list is actually expecting and then reads only
that many lines from the file.

Of course, if we'd written:

my ($name, $vers, $status, $costs, @and_the_rest) = <$fh>;

then the entire file would have been read.

And
now for something completely the same (well, almost)

Apart from the new sigil syntax (i.e. hashes now keep their %
signs no matter what they're doing), the remainder of &load_data
is exactly as it would have been if we'd written it in Perl 5.

We skip to the next file if the current file's version is wrong. Otherwise, we split the costs
line into an array of whitespace-delimited values, and then save everything
(including the still-open filehandle) in a nested hash within %data:

Then, once we've iterated over all the directories in @dirpath,
we return the accumulated data:

return %data;
}

The virtue of constancy

Perl 6 variables can be used as constants:

my @StartOfFile is const = (0,0);

which is a great way to give logical names to
literal values, but ensure that those named values
aren't accidentally changed in some other part of the code.

Writing it back

When the data is eventually saved, we'll be passing it to the
&save_data
subroutine in a hash. If we expected the hash to be a real hash variable
(or a reference to one), we'd write:

sub save_data (%data) {...

But since we want to allow for the possibility that the hash is created
on the fly (e.g. from a hash-like list of values), we need to use the slurp-it-all-up
list context asterisk again:

sub save_data (*%data) {...

From each according
to its ability ...

We then grab each datum for each file with the usual foreach ... values ...
construct:

foreach my $data (values %data) {

and go about saving the data to file.

Your all-in-one input supplier

Because the Perl 6 "diamond" operator can take an arbitrary expression
as its argument, it's possible to set a filehandle to read an entire file
and do the actual reading, all in a single statement:

my $rest = <$data.{fh}.irs(undef)>

The variable $data stores a reference to a hash, so to dereference
it and access the 'fh' entry, we use the Perl 6 dereferencing
operator (dot) and write: $data.{fh}. In practice, we could leave
out the operator and just write $data{fh}, since Perl can infer
from the $ sigil that we're accessing the hash through a reference
held in a scalar. In fact, in Perl 6 the only place you must use
an explicit . dereferencer is in a method call. But it never hurts
to say exactly what you mean, and there's certainly no difference in performance
if you do choose to use the dot.

The .irs(undef) method call then sets the input record separator of the
filehandle (i.e. the Perl 6 equivalent of $/) to undef,
causing the next read operation to return the remaining contents of the
file. And because the filehandle's irs method returns its own
invocant -- i.e. the filehandle reference -- the entire expression can
be used within the angle brackets of the read.

A variation on this technique allows a Perl program to do a shell-like
read-from-filename just as easily:

my $next_line = <open $filename or die>;

or, indeed, to read the whole file:

my $all_lines = < open $filename : irs=>undef >;

Editor's note: this document is out of date and remains here for historic interest. See Synopsis 3 for the current design information.

Seek and ye shall flatten

Having grabbed the entire file, we now rewind and truncate it, in preparation
for writing it back:

in Perl 5 and gotten back the annoying "Not enough arguments for seek"
exception. The problem is that seek expects three distinct scalars
as arguments (as if it had a Perl 5 prototype of seek($$$)), and
it's too fastidious to flatten the proffered array in order to get them.

It's handy to wrap the magical 0,0 arguments of the seek
in a single array (so we no longer have to remember this particular incantation),
but to use such an array in Perl 5 we would then have to write:

seek $data->{fh}, $StartOfFile[0], $StartOfFile[1]; # Perl 5

In Perl 6 that's not a problem, because we have * -- the list
context specifier. When used in an argument list, it takes whatever you
give it (typically an array or hash) and flattens it. So:

seek $data.{fh}: *@StartOfFile; # Perl 6

massages the single array into a list of two scalars, as seek
requires.

Oh, and yes, that is the adverbial colon again. In Perl 6, seek
and truncate are both methods of filehandle objects. So we can either
call them as:

$data.{fh}.seek(*@StartOfFile);
$data.{fh}.truncate(0);

Or use the "indirect object" syntax:

seek $data.{fh}: *@StartOfFile;
truncate $data.{fh}: 0;

And that's where the colon comes in. Another of its many uses in Perl
6 is to separate "indirect object" arguments (e.g. filehandles) from
the rest of the argument list. The main place you'll see colons guarding indirect objects
is in print statements (as described in the next section).

It is written...

Finally, &save_data has everything ready and can write the
four fields and the rest of the file back to disk. First, it sets the output
field separator for the filehandle (i.e. the equivalent of Perl 5's
$,
variable) to inject newlines between elements:

Note the use of the adverbial colon after $data.{fh} to separate the
filehandle argument from the items to be printed. The colon is required
because it's how Perl 6 eliminates the nasty ambiguity inherent in the
"indirect object" syntax. In Perl 5, something like:

In fact, the colon has an even wider range of use, as a general-purpose
"adverb marker"; a notion we will explore more fully
below.

String 'em up together

The printed arguments are: a hash slice:

$data.{qw(name vers stat)},

a stringified dereferenced nested array:

_@{$data.{costs}},

and a scalar:

$rest;

The new hash slice syntax was explained in the previous Apocalypse/Exegesis,
and the scalar is just a scalar, but what was the middle thing again?

Well, $data.{costs} is just a regular Perl 6 access to the
'costs'
entry of the hash referred to by $data. That entry contains the
array reference that was the result of splitting $costin
&load_data.

So to get the actual array itself, we can prefix the array reference
with a @ sigil (though, technically, we don't have to:
in Perl 6 arrays and array references are interchangeable in scalar
context).

That gives us @{$data.{costs}}. The only remaining difficulty
is that when we print the list of items produced by @{$data.{costs}},
they are subject to the output field separator. Which we just set to newline.

But what we want is for them to appear on the same line, with
a space between each.

Well ... evaluating a list in a string context does precisely that, so
we could just write:

"@{$data.{costs}}" # evaluate array in string context

But Perl 6 has another alternative to offer us -- the unary underscore
operator. Binary underscore is string
concatenation, so it shouldn't be too surprising that unary underscore
is the stringification operator (think: concatenation with a null string).
Prefixing any expression with an underscore forces it to be evaluated in
string context:

_@{$data{costs}} # evaluate array in string context

Which, in this case, conveniently inserts the required spaces between the
elements of the costs array.

Editor's note: this document is out of date and remains here for historic interest. See Synopsis 3 for the current design information.

A parameter by any other name

Now that the I/O is organized, we can get down to the actual processing.
First, we load the data:

my %data = load_data(filename=>'weblog', version=>1);

Note that we're using named arguments here. This attempt would blow up
badly in Perl 5, because we didn't set &load_data up to expect
a hash-like list of arguments. But it works fine in Perl 6 for two reasons:

Because we did set up &load_data with named parameters;
and

Because the => operator isn't in Kansas anymore.

In Perl 5, => is just an up-market comma with a single minor
talent: It stringifies its left operand if that operand is a bareword.

In Perl 6, => is a fully-fledged anonymous object constructor
-- like [...] and {...}. The objects it constructs are
called "pairs" and they consist of a key (the left operand of the =>),
and a value (the right operand). The key is still stringified if it's a
valid identifier, but both the key and the value can be any kind of Perl
data structure. They are accessed via the pair object's key and value
methods:

&load_data gets just two arguments, each of which is a reference
to a pair:

load_data( $pair_ref1, $pair_ref2); # Perl 6 semantics

When the subroutine dispatch mechanism detects one or more pairs as arguments
to a subroutine with named parameters, it examines the keys of the pairs
and binds their values to the correspondingly named parameters -- no matter
what order the paired arguments originally appeared in. Any remaining non-pair
arguments are then bound to the remaining parameters in left-to-right order.

The Xor-twist

Then we toggle the "is active" status bit (the eighth bit) for each file.
To flip that single bit without changing any of the other status bits,
we bitwise-xor the status bitset against the bitstring
0000000010000000.
Each bit xor'd against a zero stays as it is (0 xor 0 --> 0; 1 xor 0 --> 1),
while xor'ing the eighth bit against 1 complements it (0 xor 1 --> 1; 1 xor 1 --> 0).

But because the caret has been appropriated as the Perl 6
hyper-operator
prefix, it will no longer be used as bitwise xor. Instead, binary tilde
will be used:

%data{$file}{stat} = %data{$file}{stat} ~ $is_active_bit;

This is actually an improvement in syntactic consistency since bitwise xor (now binary ~)
and bitwise complement (still unary ~) are mathematically related:
~x
is (-1~x).

Bound aliases are particularly useful for temporarily giving a conveniently short
identifier to a variable with a long or complex name. Scalars,
arrays, hashes and even subroutines may all be given less sequipedalian
names in this way:

In our example program, we use aliasing to avoid having to write @%data{$file}{costs}
everywhere:

my @costs := @%data{$file}{costs};

An important feature of the binding operator is that the lvalue (or lvalues)
on the left side form a context specification for the rvalue (or rvalues)
on the right side. It's as if the lvalues were the parameters of an invisible
subroutine, and the rvalues were the corresponding arguments being passed
to it. So, for example, we could also have written:

my @costs := %data{$file}{costs};

(i.e. without the @ dereferencer) because the lvalue expects
an array as the corresponding rvalue, so Perl 6 automatically dereferences
the array reference in %data{$file}{costs} to provide that.

More interestingly, if we have both lvalue and rvalue lists, then each
of the rvalues is evaluated in the context specified by its corresponding
lvalue. For example:

(@x, @y) := (@a, @b);

aliases @x to @a, and @y to @b, because
@'s
on the left act like @ parameters, which require -- and bind to
-- an unflattened array as their corresponding argument. Likewise:

($x, %y, @z) := (1, {b=>2}, %c{list});

binds $x to the value 1 (i.e. $x becomes a constant),
%y
to the anonymous hash constructed by {b=>2}, and @z to
the array referred to by %c{list}. In other words,
it's the same set of bindings we'd see if we wrote:

sub foo($x, %y, @z) {...}

foo(1, {b=>2}, %c{list});

except that the := binding takes effect in the current scope.

And because := works that way, we can also use the flattening
operator (unary *) on either side of such bindings. For example:

(@x, *@y) := (@a, $b, @c, %d);

aliases @x to @a, and causes @y to bind to the
remainder of the lvalues -- by flattening out $b, @c,
and %d into a list and then slurping up all their components together.

Note that @y is still an alias for those various slurped components.
So
@y[0] is an alias for $b, @y[1..@c.length] are aliases
for the elements of @c, and the remaining elements of @y
are aliases for the interlaced keys and values of %d.

When the star is on the other side of the binding, as in:

($x, $y) := (*@a);

then @a is flattened before it is bound, so $x becomes
an alias for @a[0] and $y becomes an alias for @a[1].

The binding operator will have many uses in Perl 6 (most of which we
probably haven't even thought of yet), but one of the commonest will almost
certainly be as an easy way to swap two arrays efficiently:

(@x, @y) := (@y, @x);

Yet another way to think about the binding operator is to consider it as a
sanitized version of those dreaded Perl 5 typeglob assignments. That is:

$age := $person[$n]{data}{personal}{time_dependent}{age};

is the same as Perl 5's:

*age = \$person->[$n]{data}{personal}{time_dependent}{age};

except that it also works if $age is declared as a lexical.

Oh, and binding is much safer than typeglobbing was, because it explicitly
requires that $person[$n]{data}{personal}{time_dependent}{age}
evaluate to a scalar, whereas the Perl 5 typeglob version would happily
(and silently!) replace @age, %age, or even &age
if the rvalue happened to produce a reference to an array, hash, or subroutine
instead of a scalar.

Editor's note: this document is out of date and remains here for historic interest. See Synopsis 3 for the current design information.

Better living through sigils

We should also note that the binding of the @costs array:

my @costs := @%data{$file}{costs};

shows yet another case where Perl 6's sigil semantics are much DWIM-mier
than those of Perl 5.

In Perl 5 we would probably have written that as:

local *costs = \ @$data{$file}{costs};

and then spent some considerable time puzzling out why it wasn't working,
before realising that we'd actually meant:

local *costs = \ @{$data{$file}{costs}};

instead.

That's because, in Perl 5, the precedence of a hash key is relatively
low, so:

That's not a number...now
that's
a number!

One of the perennial problems with Perl 5 is how to read in a number. Or
rather, how to read in a string...and then be sure that it contains a valid
number. Currently, most people read in the string and then either just
assume it's a number (optimism) or use the regexes found in
perlfaq4
or Regexp::Common
to make sure (cynicism).

Perl 6 offers a simpler, built-in mechanism.

Just as the unary version of binary underscore (_) is Perl
6's explicit stringification specifier, so to the unary version of binary
plus is Perl 6's explicit numerifier. That is, prefixing an expression
with unary + evaluates that expression in a numeric context. Furthermore,
if the expression has to be coerced from a string and the string does not
begin with a valid number, the stringification operator returns NaN,
the not-a-number value.

The unary + takes the string returned by <> and converts
it to a number. Or, if the string can't be interpreted as a number, +
returns
NaN. Then we just go back and try again until we do get
a valid number.

Note that these new semantics for unary + are a little different
from its role in Perl 5, where it is just the identity operator. In Perl
5 it's occasionally used to disambiguate constructs like:

Schwartzian pairs

Another handy use for pairs
is as a natural data structure for implementing
the Schwartzian Transform. This caching technique is used when sorting
a large list of values according to some expensive function on those values.
Rather than writing:

my @sorted = sort { expensive($a) <=> expensive($b) } @unsorted;

and recomputing the same expensive function every time each value is compared
during the sort, we can precompute the function on each value once. We
then pass both the original value and its computed value to sort,
use the computed value as the key on which to sort the list, but then return
the original value as the result. Like this:

More importantly, because @costs is an alias
for @%data{$file}{costs}, when we assign the sorted list back
to
@costs, we're actually assigning it back into the appropriate
sub-entry of %data.

The ∑ of all our fears

Perl 6 will probably have a built-in sum operator, but we might
still prefer to build our own for a couple of reasons. Firstly sum
is obviously far too long a name for so fundamental an operation; it really
should be ∑. Secondly, we may want to extend the basic summation
functionality somehow. For instance, by allowing the user to specify a
filter and only summing those arguments that the filter lets through.

Perl 6 allows us to create our own operators. Their names can be any
combination of characters from the Unicode set. So it's relatively easy
to build ourselves a ∑ operator:

We declare the ∑ operator as a lexically scoped subroutine.
The lexical scoping eases the syntactic burden on the parser, the semantic
burden on other unrelated parts of the code, and the cognitive burden on
the programmer.

The operator subroutine's name is always
operator:whatever_symbols_we_want.
In this case, that's
operator:∑, but it can be any sequence
of Unicode characters, including alphanumerics:

will unexpectedly succeed (and silently too, if you run without
warnings), because the non-numeric values of both the scalars are converted to zero in
the numeric context of the ==.

But in Perl 6, non-numeric strings numerify to NaN. So, using Perl
6's multiway comparison feature, we can add an
extra != NaN to the equality test to ensure that we compared genuine
numbers.

Meanwhile, we also have to specify a precedence for each new operator we
define. We do that with the is prec trait of the subroutine. The precedence is specified
in terms of the precedence of some existing operator; in this case, in
terms of Perl's built-in unary +:

my sub operator:∑ is prec( \&operator:+($) )

To do this, we give the is prec trait a reference to teh existing operator.
Note that, because there are two overloaded forms of operator:+
(unary and binary) of different precedences, to get the reference to the
correct one we need to specify its complete signature (its name
and parameter types) as part of the enreferencing operation. The ability
to take references to signatures is a standard feature in Perl 6,
since ordinary subroutines can also be overloaded, and may need the same
kind of disambiguation when enreferenced.

If the operator had been binary, we might also have had to specify its
associativity (left, right, or non), using the
is
assoc trait.

Note too that we specified the parameter of operator:∑ with
a flattening asterisk, since we want @list to slurp up any series
of values passed to it, rather than being restricted to accepting only
actual array variables as arguments.

The implementation of operator:∑ is very simple: we just
apply the built-in reduce function to the list, reducing each
successive pair of elements by adding them.

Note that we used a higher-order function to specify the addition operation.
Larry has decided that the syntax for higher-order functions requires that
implicit parameters be specified with a $^ sigil (or @^
or %^, as appropriate) and that the whole expression be enclosed
in braces.

So now we have a ∑ operator:

$result = ∑ $wins, $losses, $ties;

but it doesn't yet provide a way to filter its values. Normally, that would
present a difficulty with an operator like ∑, whose
*@list
argument will gobble up every argument we give it, leaving no way -- except
convention -- to distinguish the filter from the data.

But Perl 6 allows any subroutine -- not just built-ins like print
-- to take one or more "adverbs" in addition to its normal arguments.
This provides a second channel by which to transmit information to a subroutine.
Typically that information will be used to modify the behaviour of the
subroutine (hence the name "adverb"). And that's exactly what we need
in order to pass a filter to ∑.

A subroutine's adverbs are specified as part of its normal parameter
list, but separated from its regular parameters by a colon:

This specifies that operator:∑ can take a single scalar adverb,
which is bound to the parameter $filter. When there is no adverb
specified in the call, $filter is default-assigned the value undef.

We then modify the body of the subroutine to pre-filter the list through
a grep, but only if a filter is provided:

reduce {$^a+$^b} ($filter ?? grep &$filter, @list :: @list);
}

The ?? and :: are the new way we write the old ?:
ternary operator in Perl 6. Larry had to change the spelling because he
needed the single colon for marking adverbs. But it's a change for the
better anyway --it was rather odd that all the other short-circuiting logical
operators (&& and || and //) used doubled
symbols, but the conditional operator didn't. Well, now it does. The doubling
also helps it stand out better in code, in part because it forces you to
put space around the :: so that it's not confused with a package
name separator.

You might also be wondering about the ambiguity of ??, which
in Perl 5 already represents an empty regular expression with question-mark
delimiters. Fortunately, Perl 6 won't be riddled with the nasty ?...?
regex construct, so there's no ambiguity at all.

Adverbial semantics can be defined for any Perl 6 subroutine.
For example:

Adverbs will probably become widely used for passing this type of "out-of-band"
behavioural modifier to subroutines that take an unspecified number of
data arguments.

Editor's note: this document is out of date and remains here for historic interest. See Synopsis 3 for the current design information.

Would you like an adverb
with that?

OK, so now our ∑ operator can take a modifying filter. How
exactly do we pass that filter to it?

As described earlier, the colon is used
to introduce adverbial arguments into the argument list of a subroutine
or operator. So to do a normal summation we write:

$sum = ∑ @costs;

whilst to do a filtered summation we place the filter after a colon at
the end of the regular argument list:

$sum = ∑ @costs : sub {$_ >= 1000};

or, more elegantly, using a higher-order function:

$sum = ∑ @costs : {$^_ >= 1000};

Any arguments after the colon are bound to the parameters specified
by the subroutine's adverbial parameter list.

Note that the example also demonstrates that we can interpolate the
results of the various summations directly into output strings. We do this
using Perl 6's scalar interpolation mechanism ($(...)), like so:

The odd lazy step

Finally (and only because we can), we print out a list of every
second element of @costs. There are numerous ways to do that in
Perl 6, but the cutest is to use a lazy, infinite, stepped list of indices
in a regular slicing operation.

In Perl 6, any list of values created with the .. operator
is created lazily. That is, the .. operator doesn't actually build
a list of all the values in the specified range, it creates an array object
that knows the boundaries of the range and can interpolate (and then cache)
any given value when it's actually needed. That's useful, because
it greatly speeds up the creation of a list like (1..Inf).

Inf is Perl 6's standard numerical infinity value, so a list
that runs to Inf takes ... well ... forever to actually build. But
writing
1..Inf is OK in Perl 6, since the elements of the resulting
list are only ever computed on demand. Of course, if you were to
print(1..Inf),
you'd have plenty of time to go and get a cup of coffee. And even then
(given the comparatively imminent heat death of the universe) that coffee
would be really cold before the output was complete.
So there will probably be a warning when you try to do that.

But to get an infinite list of odd indices, we don't want every number
between 1 and infinity; we want every second number. Fortunately,
Perl 6's .. operator can take an adverb that specifies a "step-size"
between the elements in the resulting list. So if we write (1..Inf
: 2), we get (1,3,5,7,...). Using that list, we can extract
the oddly indexed elements of an array of any size (e.g. @costs)
with an ordinary array slice:

print @costs[1..Inf:2]

You might have expected another one of those "maximal-entropy coffee" delays whilst
print
patiently outputs the infinite number of undef's that theoretically
exist after @costs' last element, but slices involving infinite
lists avoid that problem by returning only those elements that actually
exist in the list being sliced. That is, instead of iterating the requested
indices in a manner analogous to:

(Obviously, it's actually far more complicated -- and lazy -- than that.
It has to preserve the original ordering of the wanted indexes, as well
as cope with complex cases like infinite slices of infinite lists. But
from the programmer's point of view, it all just DWYMs).

By the way, binding selected array elements to the elements of another
array (as in: @slice[+@slice] := @array[$actual_index]), and then
returning the bound array as an lvalue, is a neat Perl 6 idiom for
recreating any kind of slice-like semantics with user-defined subroutines.

Take that! And that!

Note that we're passing in both a hash and a pair, but that these still
get correctly folded into &save_data's single hash parameter,
courtesy of the flattening asterisk on the parameter definition:

sub save_data (*%data) {...

In a nutshell...

It's okay if your head is spinning at this point.

We just crammed a huge number of syntactic and semantic changes into
a comparatively small piece of example code. The changes may seem overwhelming,
but that's because we've been concentrating on only the changes.
Most of the syntax and semantics of Perl's operators don't change at
all in Perl 6.

So, to conclude, here's a summary of what's new, what's
different, and (most of all) what stays the same.

Unchanged operators

prefix and postfix ++ and --

unary !, ~, \, and -

binary **

binary =~ and !~

binary *, /, and %

binary + and -

binary << and >>

binary & and |

binary =, +=, -=, *=, etc.

binary ,

unary not

binary and, or, and xor

Changes to existing operators

binary -> (dereference) becomes .

binary . (concatenate) becomes _

unary + (identity) now enforces numeric context on
its argument

binary ^ (bitwise xor) becomes ~

binary => becomes the "pair" constructor

ternary ? : bbeeccoommeess ?? ::

Enhancements to existing
operators

binary .. becomes even lazier

binary <, >, lt, gt, ==,
!=,
etc. become chainable

Unary -r, -w, -x, etc. are nestable

The <> input operator are more context-aware

The logical && and || operators propagate
their context to both their operands

The x repetition operator no longer requires listifying parentheses
on its left argument in a list context.

New operators:

unary _ is the explicit string context enforcer

binary ~~ is high-precedence logical xor

unary * is a list context specifier for parameters and a
array flattening operator for arguments